1. Field of the Invention
This invention relates generally to an apparatus and method for providing stochastic independent neural stimulation, and in particular, a neural stimulation system and method for providing pseudospontaneous activity in the auditory nerve, which can be used to treat tinnitus.
2. Related Applications
Co-pending patent application U.S. Ser. No. 09/023,279, entitled "Speech Processing System and Method Using Pseudospontaneous Stimulation", by J. Rubinstein and B. Wilson (Attorney Docket No. UIOWA-26) filed Feb. 13, 1998, containing related subject matter, is hereby incorporated by reference.
3. Background of the Related Art
Fundamental differences currently exist between electrical stimulation and acoustic stimulation of the auditory nerve. Electrical stimulation of the auditory nerve, for example, via a cochlear implant, generally results in more cross-fiber synchrony, less within fiber jitter, and less dynamic range, as compared with acoustic stimulation which occurs in individuals having normal hearing. FIG. 14 shows the magnitude of a related art pattern of electrically-evoked compound action potentials (EAPs) from an auditory nerve of a human subject with an electrical stimulus of 1 kHz (1016 pulses/s). The EAP magnitudes are normalized to the magnitude of the first EAP in the record. FIG. 14 shows the typical alternating pattern previously described in the art. This pattern arises because of the refractory period of the nerve and can degrade the neural representation of the stimulus envelope. With a first stimulus 1402 a large response occurs, likely because of synchronous activation of a large number of fibers. These fibers are subsequently refractory driving a second pulse 1404, and accordingly a small response is generated. By the time of a third pulse 1406, an increased pool of fibers becomes available (non-refractory) and the corresponding response increases. The alternating synchronized response pattern can be caused by a lack or decrease of spontaneous activity in the auditory nerve and can continue indefinitely. Variations of the alternative response pattern and more complex patterns have been observed in human (e.g., with different rates of amplitudes of stimulation), animal and modeling studies. Such complex patterns of response at the periphery may indicate limitations in the transmission of stimulus information to the central nervous system as they may reflect properties of the auditory nerve in addition to properties of the stimulus.
Loss of spontaneous activity in the auditory nerve is one proposed mechanism for tinnitus. Tinnitus is a disorder where a patient experiences a sound sensation within the head ("a ringing in the ears") in the absence of an external stimulus. This uncontrollable ringing can be extremely uncomfortable and often results in severe disability. Restoration of spontaneous activity may potentially improve tinnitus suppression. Tinnitus is a very common disorder affecting an estimated 15% of the U.S. population according to the National Institutes for Health, 1989 National Strategic Research Plan. Hence, approximately 9 million Americans have clinically significant tinnitus with 2 million of those being severely disabled by the disorder.
Several different types of treatments for tinnitus have been attempted. One related art approach to treating tinnitus involves suppression of abnormal neural activity within the auditory nervous system with various anticonvulsant medications. Examples of such anticonvulsant medications include xylocaine and lidocaine that are administered intravenously. In addition, since the clinical impact of tinnitus is significantly influenced by the patient's psychological state, antidepressants, sedatives, biofeedback and counseling methods are also used. None of these methods has been shown to be consistently effective.
Another related art approach to treating tinnitus involves "masking" undesirable sound perception by presenting alternative sounds to the patient using an external sound generator. In particular, an external sound generator is attached to the patient's ear (similar to a hearing aid) and the generator outputs sounds into the patient's ear. Although this approach has met with moderate success, it has several significant drawbacks. First, such an approach requires that the patient not be deaf in the ear that uses the external sound generator. That is, the external sound generator approach cannot effectively mask sounds to a deaf ear that subsequently developed tinnitus. Second, the external sound generator can be inconvenient to use and can actually result in loss of hearing acuity in an otherwise healthy ear.
Yet another related art approach involves surgical resection of the auditory nerve itself. This more dangerous approach is usually only attempted if the patient suffers from large acoustic neuromas as well as tinnitus. In this situation, the auditory nerve is not resected for the specific purpose of eliminating tinnitus but the auditory nerve can be removed as an almost inevitable complication of large tumor removal. In a wide series of patients with tinnitus who underwent this surgical procedure of auditory nerve resection, only 40% were improved, 10% were improved and 50% were actually worse.